This invention relates to a drive for a scanning optical system in a copying apparatus. More specifically it relates to an apparatus and method for establishing correct carriage position upon cable replacement.
In the prior art, scanning apparatus for electrophotographic copiers usually includes a full-rate carriage and a half-rate carriage. The full-rate carriage mounts a lamp and a mirror and moves along the length of a document illuminating successive portions of its surface. The half-rate carriage mounts two mirrors and moves in the same direction as the full-rate carriage, but at one half its speed. Light from the document is reflected by the full-rate mirror to the half-rate mirrors and thence to a lens. The lens focuses the light upon the photoconductive drum. The motion of the half-rate carriage relative to the full-rate carriage maintains a constant object distance from the illuminated portion of the document to the lens so that the image on the photoconductive drum is always in focus.
The two carriages are mounted on two parallel guides and are driven by a single-cable or dual-cable arrangement. In a dual-cable system, both ends of the carriages are driven. Each cable is attached to the full-rate carriage and engages a running block pulley mounted on the half-rate carriage.
For correct operation of the copying apparatus the two carriages must be parallel to each other and perpendicular to the direction of scan. Furthermore, the distance between the two carriages at any point in the scan must be repeatable from scan to scan. These parameters are set originally at the factory using sophisticated optical equipment. However, after field replacement of a broken cable the customer engineer must restore proper alignment of the optics. Up to now he has relied on precision cables and an iterative process. He replaces the cable, makes a copy and adjusts the cable at its fixed ends to compensate for an out-of-focus condition. He then makes another copy and continues the process until focus has been achieved.
It must be remembered that the customer engineer is adjusting not only the total cable length, but the ratio of that portion of the cable between the two carriages to (a) the total length, and (b) the length of cable running from the full-rate carriage to ground. He is adjusting two ratios and a total length and an adjustment of any of these three affects the other two. In a dual-cable system the problem is worse and possible combinations become staggering.
This process is extremely time consuming since the customer engineer must turn off the machine and open it up to make the adjustments and then must reclose it, turn it on and wait for it to warm up before he can make the copy. Occasionally, he is unable to readjust the focus and the carriage subassembly must be returned to the factory where it can be placed on an optical bench for readjustment of the focus. Also, the precision cables are very costly to produce. Although cutting the cables to a precise length is not difficult, the tight tolerance cannot be maintained for the process of adding the attachments at the ends of the cables.